The invention generally relates to mass spectrometers, and more particularly to optimized axial ejection techniques in a linear ion trap.
The linear ion trap is characterized by an elongate multi-pole rod set in which a two dimensional RF field is used to radially trap ions that are contained axially by a DC barrier or trapping field at an exit lens. The linear ion trap has a number of advantages over quadrupole or three-dimensional ion traps, including reduced space charge effects. Linear ion traps are described, inter alia, in U.S. Pat. No. 6,177,668 issued Jan. 23, 2001 to Hager (the xe2x80x9cHager patentxe2x80x9d), the entire contents of which are incorporated herein by reference. The Hager patent teaches a variety of axial ejection techniques, in which ions are mass-selectively scanned out of the trap by overcoming the potential barrier at the exit lens. The efficiency, sensitivity, and resolution of particular instances of the axial ejection techniques are briefly discussed.
The invention relates to improved axial ejection techniques, and in particular to maximizing the resolution of axial ejection over a wide range of ionic masses.
Broadly speaking, the invention accomplishes this by varying the DC potential barrier between the rods and the exit member of linear ion trap as a function of mass. This is carried out in conjunction with the manipulation of other fields used to axially eject ions mass-selectively. The magnitude of the potential barrier is preferably controlled to vary generally linearly as a function of ion mass-to-charge ratios (m/z), over a pre-determined m/z range. Outside the bounds of the pre-determined m/z range, the barrier field preferably remains stable.
According to one aspect of the invention an improved method of operating a linear ion trap is provided. The linear ion trap includes a DC potential barrier between the rods of the trap and an exit member adjacent to an exit end of the trap. Ions are axially ejected in the improved trap by energizing trapped ions of a selected m/z value and setting the magnitude of the potential barrier based on the selected m/z value in accordance with a pre-determined function, to thereby mass selectively eject at least some ions of a selected n/z value axially from the rod set past the exit member. In the preferred function, the magnitude of the potential barrier is substantially linearly related to the magnitude of the n/z value.
According to another aspect of the invention, there is provided a method of operating a mass spectrometer having an elongated rod set which has an entrance end, an exit end and a longitudinal axis. The method includes: (a) admitting ions into the entrance end of the rod set; (b) trapping at least some of the ions in the rod set by producing a barrier field at an exit member adjacent to the exit end of the rod set and by producing an RF field between the rods of the rod set adjacent at least the exit end of the rod set, wherein the RF and barrier fields interact in an extraction region adjacent to the exit end of the rod set to produce a fringing field; (c) energizing ions in at least the extraction region and varying a potential barrier between the exit member and rod set to mass selectively eject at least some ions of a selected mass-to-charge ratio axially from the rod set past said barrier field; and (d) and detecting at least some of the axially ejected ions. The magnitude of the potential barrier is preferably substantially linearly related to the selected ion mass-to-charge ratio.
In the preferred embodiment, an auxiliary dipole or quadrupole AC voltage is applied to the rod set to assist in axial ejection. The population of ions contained by the linear ion trap is preferably axially ejected therefrom by simultaneously ramping or scanning the RF field, the auxiliary AC field and the DC voltage on the exit lens (or alternatively or additionally a DC offset voltage applied to the rod set). The ions may thus be axially ejected orderly by increasing or decreasing m/z values, depending on the direction (upward or downward) of the ramping, thereby facilitating a mass scan or the collection of mass spectra.